How do I add a png-image on top of an image in the book module?
No such combining of images is possible in Lightroom.
You may first edit the image in some external editor like Photoshop by using the Edit In option in Lightroom and then can use the same image in your Book.
Refer: Adobe Photoshop Lightroom Help | Open and edit Lightroom photos in Photoshop or Photoshop Elements
Photoshop Help | Combine multiple images into a group portrait
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